Wide-band transformer having neutralizing winding



: Jar 1. 17, 1967 HUA-TUNG E 3,299,384

WIDE-BAND TRANSFORMER HAVING NEUTRALIZING WINDING Filed July 1, 1964 s \NF IYNVENTOR HUA-TUNG LEE ATTORNEY United States Patent 3,299,384 WIDE-BAND TRANSFORMER HAVING NEUTRALIZING WINDING Hua-Tung Lee, Olive, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a

corporation of New York Filed July 1, 1964, Ser. No. 379,644 4 Claims. (Cl. 336-171) This invention relates to signal translating devices and more particularly to wide band coupling arrangements.

Wide-band transformers (up to 200 me. bandwidth) are often made with interlaced primary and secondary windings in order to reduce the leakage flux between them. The close proximity of the windings increases the distributed capacitive coupling between the primary and a secondary windings. For an unbalanced-to-balanced transformer, this capacitive coupled voltage adds vectorially to the inductively coupled secondary voltages, and the resultant secondary voltages, are in general, not balanced.

Electrostatic shields between primary and secondary windings have been used in the design of narrower band transformers to reduce capacitive effects. This technique is impractical for the high frequency wide-band transformer design because of the mechanical features and the undesirable increased winding to ground capacitance introduced by such a shield.

The present invention provides a wide-band transformer'wit-h a neutralizing winding which cancels the distributed capacitive coupling between primary and secondary windings.

In preferred embodiments of the invention, the neutralizing winding has essentially the same number of turns as the primary winding and is wound retrogressively with respect to it along the core. One end of the neutralizing winding is connected to one end of the primary and to ground, and the other end is floating. This enables a simple construction while providing for a constant net electric field with respect to ground.

Accordingly, it is an object of the invention to provide an improved wide band network device.

It is another object of the invention to provide an improved network device as aforedescribed which functions as an unbalanced-to-balanced transformer in an improved manner.

It is yet another object of the invention to provide a transformer as aforesaid of compact design.

It is still another object of the invention to provide a transformer as aforedescribed in which close coupling can be achieved without the disadvantage of capacitative coupling effects.

-It is still another object of the invention to provide an improved wide band transformer as aforesaid employing dynamic shielding which is operative with a minimal energy loss.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawmg.

FIG. 1 is a schematic diagram of a transformer in accordance with the invention;

FIG. 2 is a diagrammatic showing of a preferred physical arrangement of the core and two of the windings of the transformer of FIG. 1;

FIG. 3 is a vector diagram applicable to operation of a transformer constructed in accordance with FIGS. 1 and 2;

FIG. 4A is a diagram similar to that of FIG. 2, wherein the structure has been developed into a straight-line axis of purposes of explanation of the operation of the invention; and

3,239,384 Patented Jan. 17, 1967 FIG. 4B is potential versus axial location diagram pertinent to operation of the invention in accordance with the preceding figures.

Referring more particularly to the drawing, FIG. 1 shows the winding connection arrangement of an unbal-anced-to-balanced transformer in accordance with a preferred embodiment of the invention. In that arrangement, the transformer core 8 has a primary winding 10 which has grounded and ungrounded input terminals 12, 14 respectively. The ends of this winding are also identified in the diagram by the indicia P and P signifying start and finish ends of the winding as described hereinafter. The illustrated transformer also has two secondary windings 16, 18 which are connected in series as indicated at 20 to constitute a balanced secondary arrangement having terminals 22, 24 and a center tap terminal 26. The winding direction of the secondary windings 16, 18 are the same, as indicated by the dots 30, 32. The winding direction of the primary winding can be the same or opposite from that of the secondary windings; in the present instance it is shown as being in the same direction as shown by the dot 34.

The winding arrangement as thus far described is conventional. In accordance with the prior art various schemes have been employed to overcome the difficulty presented by such an arrangement in a high frequency transformer in that there is considerable capacitative coupling between the primary and the secondary windings whereby each of the halves of the secondary receives an electric field signal from the primary and these signals tend to make the secondary output unbalanced. One prior art arrangement for overcoming this difficulty is to interpose a grounded foil shield between the primary winding 10 and the secondary windings 16, 18, but this arrangement introduces a very large amount of capacity to ground.

In accordance with the present invention a neutralizing winding 36 is provided which generates, in effect, a dynamic shield which prevents substantial electric field coupling from the primary to the secondary windings. The neutralizing winding 36 is wound retrogessively (oppositely) with respect to the primary winding 10 as indicated by the polarity dot 37, and is substantially an open circuit element, that is, has at least One end 38 which is not connected in the transformer network. In the preferred embodiment illustrated, that end is not connected at all, so that no current flows in the winding 36.

Accordingly, the neutralizing winding 36 has no substantial effect on the magnetic field in the transformer and absorbs no substantial energy. Nonetheless, it has induced in it a potential gradiant which is equal and opposite to that of the primary, by operation of the magnetic field of the primary in the core 8.

Moreove'r, in accordance with a preferred embodiment of the invention the other end 40 of the neutralizing winding 36 is connected as indicated at 42 to the adjacent grounded end of the primary 10. Accordingly, the potential status of the neutralizing win-ding, turn for turn is equal in amplitude and opposite in sign to that of the primary, with respect to ground.

The winding arrangement of the primary and neutralizing windings with respect to the core 8 is illustrated in FIG. 2. As shown in that figure, the core 8 preferably has a toroidal form, and the primary winding 10 may be placed thereon by starting at the end market P and winding in a counterclockwise direction around the core, pro gressing in a counterclockwise direction along the length of the core to the end P Similarly the neutralizing winding 36 may be wound by starting adjacent the end P of the primary and winding in a clockwise direction around the core, progressing in a clockwise direction along the core to the end N adjacent the end P For clarity, the secondary windings 16, 18 may have been omitted from the showing of FIG. 2. However, they may be provided by a single bifilar winding wound in the same direction and with the same progression .as the primary winding 10. The neutralizing 36 may occupy a geometric position between the primary and secondary windings, but it is preferred in practice to apply the neutralizing winding last since this permits a more compact winding structure and facilitates turns adjustment as hereinafter described.

Referring now to FIG. 3, it will be seen that when a sinusoidal signal is applied to the input terminals 12,14 of the primary, the electric vector E applied to the transformer will set up a magnetizing current I Assuming a low loss design, the vector I will trail that electric vector by nearly 90. The magnetic fiux resulting from the magnetizing vector I will, in turn, set up an electric vector E in the neutralizing winding 36, virtually diametrically opposed to the primary vector E This relationship is shown in another way in FIGS. 4A and 4B, In FIG. 4A, the showing of FIG. 2 has been developed, that is, unrolled to a straight line for purposes of explanation. In FIG. 4 directly below, plots 44, 46 are shown for the peak potentials E in the primary and neutralizing windings, respectively, versus distance D along the axis 48 of the core 8, at a given instant. One quarter cycle later these two potential curves pass each other on the center line or zero potential value. Another quarter cycle later they will have adopted a value exactly opposite to that which they had in the first place. It will be seen that in every case the average value, of the two curves is that if the horizontal zero potential axis 50.

The introduction of a neutralizing winding reduces the effects of capacitive coupled voltage in the secondary of a wide-band, high frequency transformer.

The neutralizing Winding turns can be adjusted to give the best balance. This is a great convenience since any mechanical unbalance between the two halves of the secondary or between the primary and neutralizing winding can be easily compensated.

In one example, a transformer in accordance wit-h the invention has the following characteristics:

Input and output impedance: 75 ohms Power handling capacity: no less than 10 mw.

Upper operating frequency limit: greater than 140 me. Insertion loss: less than 1.0 db Amount of secondary unbalance: less than 2.5% Transformer core: Ferrite toroid GA" O.D.)

Primary winding: 13 turns a. W.G.

Each /2 secondary winding: 13 turns #30 a. W.G. Neutralizing winding: 1114 turns #30 a. W.G.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a transformer,

a magnetic circuit comprising a toroidal core,

a primary winding on said core having input terminal means,

a neutralizing winding on said core geometrically coextensive with said primary winding,

said neutralizing winding being Wound oppositely with respect to said primary winding,

one end of said neutralizing winding being substantially open circuited,

and secondary winding means on said core substantially geometrically coextensive with said primary and neutralizing windings,

whereby said neutralizing winding is operative to offset in its effect on said secondary winding means an electric potential applied to said primary winding by said input means.

2. In an unbalanced-to-balanced transformer,

a magnetic circuit comprising a toroidal core,

a primary winding on said core 'having grounded and ungrounded input terminals,

a neutralizing winding on said core geometrically coextensive with said primary winding,

said neutralizing winding being wound oppositely with respect to said primary winding and the grounded end of said primary winding being connected to the adjacent end of said neutralizing winding,

the other end of said neutralizing winding being open circuited,

and a center tapped secondary winding on said core comprising winding halves which are each geometrically coextensive with said primary and neutralizing windings.

37in a transformer,

a magnetic circuit assembly comprising:

a first winding having terminal means,

a neutralizing winding geometrically coextensive with said first winding,

said neutralizing winding being wound oppositely with respect to said first winding,

one end of said neutralizing winding being substantially open circuited,

and second winding means substantially geometrically coextensive with said first and neutralizing windings,

whereby said neutralizing winding is operative to offset in its effect On said second winding means an electric potential applied to said first winding.

4. In a transformer,

a magnetic circuit assembly comprising:

a first winding having grounded and ungrounded terminals,

a neutralizing winding geometrically coextensive with said first winding,

said neutralizing winding being wound oppositely with respect to said first winding and the grounded end of said first winding being connected to the adjacent end of said neutralizing winding,

the other end of said neutralizing winding being open circuited,

and a center tapped second winding comprising winding halves which are each geometrically coextensive with said first and neutralizing windings.

References Cited by the Examiner UNITED STATES PATENTS 2,412,609 12/1946 Ganz 336 2,692,372 10/ 1954 Goldstine. 2,700,129 1/1955 Guanella. 2,860,312 11/1958 Krepps 336-69 LARAMIE E. ASKIN, Primary Examiner.

0 ROBERT K. SCHAEFFER, Examiner.

D. I. BAKER, T. I. KOZMA, Assistant Examiners. 

3. IN A TRANSFORMER, A MAGNETIC CIRCUIT ASSEMBLY COMPRISING: A FIRST WINDING HAVING TERMINAL MEANS, A NEUTRALIZING WINDING GEOMETRICALLY COEXTENSIVE WITH SAID FIRST WINDING, SAID NEUTRALIZING WINDING BEING WOUND OPPOSITELY WITH RESPECT TO SAID FIRST WINDING, ONE END OF SAID NEUTRALIZING WINDING BEING SUBSTANTIALLY OPEN CIRCUITED, AND SECOND WINDING MEANS SUBSTANTIALLY GEOMETRICALLY COEXTENSIVE WITH SAID FIRST AND NEUTRALIZING WINDINGS, WHEREBY SAID NEUTRALIZING WINDING IS OPERATIVE TO OFFSET IN ITS EFFECT ON SAID SECOND WINDING MEANS AN ELECTRIC POTENTIAL APPLIED TO SAID FIRST WINDING. 